1. Field of the Invention
The present invention relates to a multi-layer aircraft adhesive useful as an aircraft hump seal and also to a method for applying a multi-layer aircraft adhesive to an aircraft transparency, such as an aircraft window or windshield.
2. Technical Considerations
Aircraft windshields are as important to the structure of the fuselage as are the aluminum or composite panels that make up the rest of the pressurized aircraft vessel. Because they face forward, windshields must be able to protect the flight crew from impacts from birds and other objects, and they have to be sufficiently transparent and relatively free from optical distortion so as not to impair vision therethrough. Windshields, whether plastic and/or glass, typically are laminated structures formed by windshield panels bonded together by an interlayer. The number and shape of windshield panels depends on the requirements of the airframe designer. Generally, the outermost panel covers a windshield de-icing film that is electrically connected to a bus bar assembly. The interlayers between the panels are generally made of vinyl or urethane, or a mixture thereof. A delamination in the strict sense of the word is a failure of the interlayer between two panels. Delamination can be clear where panels separate or debond completely, or delamination can be cloudy when moisture creeps in between the panels. Delamination is only one example of undesirable occurrences that can damage the windshields. Windshields can also be damaged by bubble formation, discoloration, electrical de-icing failure, scratches, moisture seal erosion, interlayer degradation and peel chipping.
Usually, the first step in an aircraft windshield failure is a loss of integrity of the moisture seals or “hump seals”. Hump seals are formed over seams or interfaces between adjacent components, such as between the windshield and the adjacent metal framework. An aged or eroded hump seal often is the culprit. Moisture can seep into and corrode the urethane interlayer, which can lead to debonding at the glass/interlayer interface. If hump seal maintenance is not done, additional moisture can corrode the bus bar-to-deicing film junction interface. This corrosion can create microscopic breaks in the conductor path that can produce arcing and ultimately result in the fracture of the outboard glass panel. The appearance of delamination and/or discoloration in the edge of a windshield usually means that the hump seal is failing. If the seal is repaired immediately, the delamination can usually be stopped.
In aircraft manufacture and maintenance, the hump seals typically include only one adhesive that blocks moisture vapor transmission to the susceptible interlayers between the glass or plastic plies. The adhesive also provides abrasion resistance to air, dust, and/or rain exposure. Typical adhesives used for hump seals in aircraft transparencies are polysulfide type or silicone-based adhesives having slow curing rates and poor low temperature properties.
Therefore, a need exists to provide a multi-layer aircraft adhesive for aircraft transparencies, such as windshields and windows, that is fast curing, provides improved moisture vapor transmission blockage, and improved abrasion resistance.